Diagnosis of an autoimmune disease generally is done utilizing, in addition to direct determination based on the symptoms etc. of a patient, indirect determination based on the detection of an autoantibody specific to the autoimmune disease. The detection of an autoantibody usually is performed using, e.g., an ELISA method in which a purified antigen protein is immobilized on a carrier and the binding of the autoantibody with the immobilized protein is examined. However, even when patients are diagnosed with an autoimmune disease by doctors on the basis of their symptoms, the autoantibody is not necessarily detected in all the patients by the ELISA method. Thus, if diagnosis is made depending on the detection of the autoantibody by the ELISA method alone, false negative results may be obtained. Therefore, for diagnosis of an autoimmune disease, a determination method other than the autoantibody detection by the ELISA method has to be used in combination. Under these circumstances, in order to enable highly reliable diagnosis of autoimmune diseases, there are demands for a method that can detect an autoantibody with high accuracy.